(a) Field of the Invention:
The present invention relates to a fuel injection device for injection carburetors capable of accurately controlling fuel injection rates adequately on the basis of negative pressures produced in accordance with flow rates of air to be sucked.
(b) Description of the prior art:
As an example of the conventional fuel injection devices of this type, there is known the Bendix-Stromberg fuel injection system for aircrafts (see "Spark-Ignition Engines: Fuel Injection Systems", CHARLES H. FISHER, CHARLES & HALL, 1966. The structure and functions of this fuel injection system will be briefly described below. The fuel injection system comprises a fuel control unit consisting of an air pressure regulator divided by an air diaphragm into an atmosphere chamber into which impact pressures are to be introduced and a depression chamber into which venturi pressures are to be introduced; a fuel pressure regulator divided by a fuel diaphragm into a fuel pressure chamber into which a fuel is fed by a fuel pump and a fuel injection chamber communicated with the fuel pressure chamber through a metering jet; a connecting member which is connected to the air diaphragm and the fuel diaphragm, and has a poppet valve for opening and closing a fuel inlet to the fuel pressure chamber; and a fuel nozzle which is connected to the fuel injection chamber and functions to eject the fuel into a suction tube at a location downstream a throttle valve. When a flow rate of air to be sucked through the throttle valve is set in this fuel control unit after the engine is started, a differential pressure corresponding to the air flow rate is produced between the atmosphere chamber and the depression chamber of the air pressure regulator, and the air diaphragm displaces the connecting member toward the depression chamber so as to open the poppet valve widely. On the other hand, the differential pressure between the fuel pressure chamber and the fuel injection chamber is applied to the fuel diaphragm, which displaces the connecting member in the reverse direction so as to close the poppet valve. At a position where these two differential pressures acting in the reverse directions are balanced with each other, opening degree of the poppet valve is determined, whereby flow rate of the fuel to be introduced into the fuel pressure chamber is determined. Since the fuel nozzle discharges the fuel at a constant pressure, the fuel pressure downstream the metering jet is set at a predetermined level proportional to the nozzle pressure. Accordingly, there is produced between the upstream side and the downstream side of the metering jet a differential pressure which is equal to the differential pressure produced between the atmosphere chamber and the depression chamber of the air pressure regulator. Since both the air flow rate and the fuel flow rate are proportional to the root of the above-mentioned differential pressure, the flow rate of the fuel to be ejected into the suction tube is proportional to the flow rate of air to be sucked and air-fuel ratio is kept constant.
However, this fuel injection device is practically capable of controlling flow rate of air to be sucked and flow rate of the fuel to be ejected only within a range of 8 to 10 times of the minimum flow rates thereof. When the negative pressure produced in the venturi by the air flow rate for the maximum output is assumed to be approximately 30 mmHg, for example, the negative pressure produced by the minimum controllable air flow rate is 0.3 mmHg (4.1 mmAq) and the fuel injection device is incapable of controlling air flow rates lower than 0.3 mmHg. On the other hand, the usual engines for automobiles must have a capability to control air flow rates and fuel flow rates within a range of about 60 times of the minimum flow rates. Therefore, the fuel injection system described above cannot be used as a fuel injection device for automobiles.